Electrolytic etching bath



Patented May 27, .1947

ELECTROLYTIC ETCHING BATH Kenneth J. Carson, North Caldwell, and Justin M. Kelly, East Orange, N. J assignors to Western Electric Company, Incorporated, New York, N. Y., a corporation of New York No Drawing. Application January 4, 1944," Serial No. 516,982

1 Claim. 1

This invention relates to an etching reagent, and more particularly to a reagent especially adapted for etching metallographic samples of certain alloys of nickel.

Considerable use is made in various electrical arts of alloys of nickel and especially of the various alloys known as Permalloy which consist essentially of nickel and iron or of nickel and iron with minor amounts of other metals, notably chromium or molybdenum. It is found that when metallographic samples of these as well as of other nickel base alloys, e. g. cupronickel, Monel metal, and the like, are prepared, especially for microscopic and photomicrographic examination, the etching of the surface of such a sample to develop the structure detail is a difficult matter. Apparently most of the known etching solutions, whether used with or without electrolytic aid, either are ineffective or else attack the material uncontrollably and produce excessive pitting or grain boundary attack. This is particularly true in the case of the nickeliron-molybdenum Permalloy.

An object of the present invention is to provide an etching reagent particularly effective to produce controllable and satisfactory electrolytic etching of metallographic samples of such nickel base alloys.

With the above and other objects in view, the invention may be embodied in an electrolytic etching reagent for preparing for metallographic examination samples of nickel base alloys and comprising only hydrochloric acid and ferric chloride as principal etching agents, together with a dichromate and acetic acid as sole stain preventing agents.

Other objects and features of the invention will appear from the following detailed description of an embodiment thereof.

A solution is made up by mixing, preferably, about 62 parts by volume of concentrated (38%) hydrochloric acid with about 35 parts by volume of a solution of ferric chloride in Water.

While the solution of ferric chloride acidified with hydrochloric acid above described is operative, in many instances satisfactorily, there are instances, especially where the Permalloys are concerned, where the etching is satisfactory but staining of the grains developed by the etching becomes troublesome. In such instances the addition of small amounts of dichromate, e. g. of a 3% solution of potassium dichromate in water acidulated with sulfuric acid, together with a small amount of acetic acid, produces a bath for electrolytic etching which produces controllable,

2 clean out and clean etching of substantially any nickel base alloy.

A preferred formula for the preparation of the modified reagent is as follows:

Concentrated (38%) hydrochloric acid parts by volume 62 10% ferric chloride in water do 35 Glacial acetic acid do 2 Solution II do 1 Water cubic centimeters Potassium dichromate grams 3 Concentrated sulfuric acid (oil of vitriol) cubic centimeters" 3 sodium chloride in water do 1 This, it is to be noted, is believed to represent the optimum proportions for use as reagent in the electrolytic etching of molybdenum Permalloy. These proportions may be varied considerably, especially for other nickel base alloys, and still be satisfactory.

In operation, a specimen sample is prepared by mechanically polishing a-suitable portion of its surface, then making the sample the anode in an electrolytic bath of the above described reagent for from 10 to seconds with a direct current of l to 3 volts, and washing and drying the sample. The length of time and the voltage required will vary within the above limits according-to the particular alloy in question and also with the degree of mechanical polish given the surface to be etched. In the case of molybdenum Permalloy, one of the most difficult of nickel base alloys to etch satisfactorily, use of the last given preferred formula above for 10 to 20 seconds at about 1.5 volts effects a clean and clear etch.

What is claimed is:

As a bath for the electrolytic etching of samples of nickel base alloys in preparation for metallographic examination, a water solution consisting of about 62 parts by volume of 38% hydrochloric acid, about 35 parts by volume of 10% solution of ferric chloride in water, about 2 parts by volume of glacial acetic acid, combined with about one part by volume of a water solution proportioned to contain about 3 grams of an alkali dichromate, about 3 cubic centimeters of oil of vitriol and about 0.10 gram sodium chloride in each 100 cubic centimeters of water.

KENNETH J. CARSON. JUSTIN M. KELLY. (References on following page) REFERENCES CITED FQREIGN PATENTS The following references are of record in the Number y Da e file of this patent: 5 ,7 Great Brltaln 1905 OTHER REFERENCES ED STATES FATE Metal Finishing, Dec. 1942, lefthancl column of Number Name Date page 657. (Copy in Scientific Library.)

1,289,022 Weeks Dec. 24, 1918 Nickel and Its Alloys, Bureau of Standards 1,291,310 Weeks Jan. 14, 1919 10 Circular No. 100 (1924), pp. 23, 55 and 72. (Copy 1, 1 ,840 Weeks Sept. 2, 1919 in Div. 3.)

1,903,778 Conroy Apr. 18, 1933 Sauveur, The Metallography of Iron and Steel 2,080,348 Truitt May 11, 1937 published 1938 by McGraw-Hill, page 487. (Copy 2,154,027 Brennan Apr. 11, 1939 in Div. 3.)

2,182,244 Beall Dec. 5, 1939 I5- Metals Handbook published 1939 by the Ameri- 2,22 ,381 Norris Dec. 24, 1940 can Society for Metals, p. 1676. (Copy in Div, 3.) 

